Understanding Your Liver Function Tests (LFTs)

A Patient Information Guide

Your liver is one of the hardest-working organs in your body, quietly performing over 500 vital roles—from filtering toxins and processing medications to converting nutrients into energy. When your doctor requests a Liver Function Test (LFT) panel, they are checking a specific snapshot of enzymes and proteins in your bloodstream.

Ironically, the term “liver function tests” is a bit of a misnomer; most of these markers actually look for signs of liver cell irritation or bile duct inflammation rather than measuring how well the organ is working overall. The information below explains the most common markers on an LFT panel and what your results actually mean.

Alanine Aminotransferase (ALT)

Typical Normal Range: 5–40 IU/L

ALT is an enzyme found almost exclusively inside your liver cells. It plays a foundational role in metabolic health by helping your liver break down proteins into usable cellular energy.

• Elevated ALT: When liver cells are actively irritated, inflamed, or physically injured, they leak this enzyme into your bloodstream. Mild elevations are incredibly common and frequently caused by non-alcoholic fatty liver disease (NAFLD), moderate alcohol intake, or a temporary reaction to a new medication (like statins or antibiotics).

• Very High ALT: Levels that spike into the hundreds or thousands usually signal acute, temporary liver injury, such as a new viral hepatitis infection or toxic strain from paracetamol over-use.

Aspartate Aminotransferase (AST)

Typical Normal Range: 5–40 IU/L

AST is another cellular enzyme that acts very similarly to ALT, but with one key structural difference: while ALT is mostly unique to the liver, AST is also found in significant amounts inside heart tissue, skeletal muscles, and red blood cells.

• Elevated AST: Your doctor will always evaluate AST alongside your ALT to look for patterns. If both are elevated, it strongly points toward a primary liver issue.

• The AST/ALT Ratio: The relationship between these two numbers provides useful diagnostic clues. For instance, a pattern where ALT is higher than AST is standard in early fatty liver disease, whereas an AST level that is double the ALT level can sometimes point to alcohol-related liver irritation.

Alkaline Phosphatase (ALP)

Typical Normal Range: 30–130 IU/L

ALP is a specialized metabolic enzyme located primarily on the walls of your liver’s bile ducts—the complex network of drainage tubes that transport bile into your digestive tract to help you digest fats. It is also found abundantly in bone tissue.

• Elevated ALP: A raised ALP level typically flags a “cholestatic pattern,” meaning there is an issue with bile flow. This is most commonly caused by a physical blockage in the drainage tubes, such as a gallstone, or inflammation of the bile ducts themselves.

• Non-Liver Causes: Because ALP is heavily present in bone, a raised level can also happen entirely outside the liver during a pregnancy, after a bone fracture heals, or in conditions like osteoporosis.

Gamma-Glutamyl Transferase (GGT)

Typical Normal Range: 0–30 IU/L (Women) | 0–50 IU/L (Men)

GGT is a highly sensitive transport enzyme found throughout your liver’s bile duct system. It is often used by clinicians as a “tie-breaker” or confirming test.

• Elevated GGT: If you have a high ALP level and your GGT is also high, it completely confirms to your doctor that the issue is originating within your liver or bile ducts (rather than your bones).

• Sensitivities: GGT is notoriously sensitive to lifestyle factors. Even a minor increase in regular alcohol intake or taking certain over-the-counter medications can cause GGT to jump up, serving as an early indicator that the liver is working harder to clear substances from your system.

Bilirubin

Typical Normal Range: 0–20 µmol/L (0.1–1.2 mg/dL)

Bilirubin is a natural, yellow-green pigment produced when your body breaks down old, worn-out red blood cells. Under normal conditions, your liver extracts this raw pigment from your blood, chemically processes it, and safely excretes it via bile into your stool.

• Elevated Bilirubin: If your liver cells are struggling or your bile ducts are blocked, bilirubin accumulates in the bloodstream. Once levels rise significantly, it causes jaundice—a noticeable yellowing of the skin and the whites of the eyes.

• Gilbert’s Syndrome: If your bilirubin is slightly elevated but every other single marker on your liver panel is pristine, it is highly likely you have a completely harmless, genetic quirk called Gilbert’s Syndrome. This affects up to 8% of the population and requires no treatment whatsoever.

Albumin

Typical Normal Range: 35–50 g/L

Unlike the enzymes listed above (which leak out during cell damage), albumin is a vital structural protein made exclusively by a healthy liver. It acts like a cellular sponge, keeping fluid from leaking out of your blood vessels into surrounding tissues, while transporting hormones and vitamins through your body.

• Low Albumin: A drop in albumin reflects your liver’s actual synthetic capacity (its production capability). Low levels can show that the liver is under chronic, long-term strain and struggling to build proteins.

• Other Influences: Because albumin is so reactive to overall body fluid, low levels can also be caused by systemic inflammation, nutritional deficiencies, or kidney issues where protein is accidentally lost in the urine.

Clotting Markers (Prothrombin Time / INR)

Typical Normal Range: INR around 1.0 (PT: 10–13 seconds)

While not always grouped directly on the standard automated biochemical chemistry panel, a Prothrombin Time (PT) or International Normalised Ratio (INR) test is a vital component of checking overall liver health. This test measures exactly how many seconds it takes for your blood to clot.

• Delayed Clotting (High INR): The liver is solely responsible for producing the specialized proteins (clotting factors) that let your blood knit together after an injury. Because these clotting proteins have a very short lifespan, a sudden prolongation in your clotting time is one of the most reliable and sensitive indicators of real-time liver strain or function changes.

Common Causes of Slightly Abnormal Results

If you log onto your patient portal and spot a marker slightly outside the lab’s reference range, try not to panic. Statistically, roughly 5% of perfectly healthy individuals will fall just outside the mathematical “normal range” purely by chance.

The vast majority of mildly abnormal LFTs are managed perfectly in primary care without requiring a specialist referral. The most common underlying drivers include:

• Metabolic Changes: Extra fat accumulation in the liver cells (NAFLD) driven by weight, insulin resistance, or diabetes.

• Medications & Supplements: Everyday drugs like paracetamol, ibuprofen, statins, or certain herbal supplements can cause temporary fluctuations.

• Recent Illness: A simple viral bug, flu, or recent intense physical exercise can temporarily shift your numbers.

How Often Should Your Liver Blood Tests Be Checked?

There is no universal, one-size-fits-all timeline for monitoring your liver health. The frequency is dictated entirely by your personal medical profile:

• Routine Health Screens: Checked every 1 to 3 years during routine wellness evaluations or standard health MOTs.

• Medication Monitoring: If you are starting a high-risk long-term drug, your GP may check your bloods at 3 months, 6 months, and then annually to ensure your liver tolerates the treatment perfectly.

• Investigating Abnormalities: If an initial test shows unexpected irritation, your doctor will generally recommend lifestyle adjustments and schedule a repeat test in 1 to 3 months to check if the trend resolves naturally.

How to Monitor Your Liver Health Over Time

A single, isolated blood test result provides very little context compared to tracking your individual trends over several months or years.

To view and track your lab data as soon as it leaves the hospital laboratory, MyHSN suggests using secure online platforms such as Patients Know Best (PKB) or the NHS App.

Keeping an eye on your numbers allows you to see how positive lifestyle updates—such as moderating alcohol intake, maintaining a balanced diet, and staying active—directly reduce liver stress and support your long-term health.